Anechoic chamber calibration using sweep signals

Abstract

ISO 26101-1:2021 specifies a standard method for qualifying a space as being anechoic. The method involves placing a source within the chamber and checking whether the sound pressure decays in accordance with the inverse square law at different frequencies and in different directions. In the method adopted in this study, a microphone was traversed along a wire away from the source and the sound pressure level measured as the microphone traversed. The standard recommends using pure tones at the centre frequencies of the standard one-third octave bands as the noise signal. In order to speed up testing, multiple tones can be played simultaneously. However, using multiple tones which includes tones at harmonics of others results in beating which affects the measured sound pressure level over the short period required when using a traversing microphone. This article describes how a swept sine signal was used instead during the calibration of the anechoic chamber at the University of Auckland. The results obtained using the proposed method were compared with those obtained using single pure tones and the results were observed to be consistent. It is also noted that the measured cut-off frequency of the anechoic chamber at the University of Auckland is below 50 Hz.